There is currently a significant level of interest in extending the capabilities of infrared imaging systems beyond conventional amplitude imaging to include polarimetry which provides additional information which can be used to highlight or suppress different materials in a scene, or objects at different orientations. Polarimetry can be useful, for example, to distinguish man-made objects in a scene from natural objects. The usual method for imaging polarimetry is to sequentially measure each polarization component of an image, and then to use these measurements to calculate the Stokes parameters in order to characterize the polarization state of each pixel in the image. However, this sequential method does not allow polarization-sensitive imaging of dynamically-changing scenes due to movement of the imaging system, or due to movement of objects within the scene, or both.
A number of different approaches have been developed for imaging polarimetry as detailed in U.S. Pat. Nos. 5,890,095; 7,173,698; 7,186,968; and in U.S. Pat. Appl. Pub. 2007/0241267. These approaches have met with limited success.
The present invention provides an advance in the art by providing a polarization-sensitive infrared imaging sensor which utilizes fiber optics to reduce the cross-talk between adjacent pixels of an infrared image in the sensor, and thereby improve an extinction ratio of the sensor. The sensor of the present invention utilizes a two-dimensional (2-D) array of polarizers which are fabricated directly on the end of a plurality of optical fibers. The optical fibers can be formed as a fiber optic faceplate. A 2-D array of infrared phase retarders can be formed on a substrate and attached directly to the fiber optic faceplate to minimize the distance between the retarders and the polarizers to minimize the distance therebetween and thereby minimize diffraction effects which could otherwise result in crosstalk between adjacent pixels of the infrared image in the sensor. The fiber optic faceplate can be attached directly onto a focal plane array to form a compact and rugged integrated unit, with the optical fibers guiding infrared light to photodetectors in the focal plane array while minimizing the cross-talk between adjacent pixels of the infrared image.
The polarization-sensitive infrared imaging sensor of the present invention allows polarization information to be extracted from the infrared image in real-time, and allows the polarization information to be presented in parallel with intensity information. This is useful to form a super pixel image of a scene in which all four of the Stokes parameters are simultaneously measured, thereby eliminating noise which could otherwise occur due to scene movement if sequential measurements of the Stokes parameters were to be made.
These and other advantages of the present invention will become evident to those skilled in the art.